It really depends on what’s causing that error message to appear. If it simply can’t boot due to a corrupted boot sector then you could try the following:
Plug the “bad” hard disk into another computer that already has Windows 2K or XP running. You should be able to see the data assuming it’s not already corrupted. If you can see it then dump as much data as you can over to the other hard disk.
or
You could also try to “repair” the current installation assuming you have the OS install discs or install a “parallel” version of the OS on that same hard disk. For instance, instead of reinstalling the OS to the same C:\WINDOWS directory, you could use C:\WINXP. That will leave the existing files alone and install a fresh copy in another directory.
I prefer the first option first since it’s less messy but it assumes you have another system laying around.
Of course, if the disc is physically failing then these techniques may not be that useful. See what works better for you and let us know what happens.